One conventional method of mounting the blades of a helicopter rotor on the rotor hub is to provide an articulated blade retention system between each rotor blade and the hub. Early proposals for articulated rotor blade retention systems incorporated antifriction bearings, such as roller bearings or ball bearings, to provide the necessary articulation. More recently, articulated retention systems have been developed which incorporate one or more laminated elastomeric bearings, rather than antifriction bearings. Such laminated bearings comprise a plurality of alternating and bonded together layers of elastomeric material and a substantially inextensible material. The bearings do not require lubrication, have only limited maintenance requirements and afford a reduction in weight as compared to antifriction bearings. The advantages of laminated elastomeric bearings are such that the bearings have found widespread use and acceptance throughout the helicopter industry and have been incorporated in a number of different helicopter rotor blade retention systems.
One fully articulated blade retention system that incorporates laminated elastomeric bearings is described and illustrated in a number of related patents assigned to United Aircraft Corporation (now United Technologies Corporation). The patents are Rybicki U.S. Pat. No. 3,759,631, Rybicki U.S. Pat. No. 3,759,632, Rybicki U.S. Pat. No. 3,782, 854, Rybicki et al U.S. Pat. No. 3,764,230 and Rybicki et al U.S. Pat. No. 3,829,239. As shown in each of the Rybicki patents, the United Aircraft retention system secures each rotor blade to a rotor hub by two serially interconnected, laminated elastomeric bearings. One laminated bearing, which has annular, disc-shaped laminations, resists centrifugal loads on the rotor blade and accommodates oscillatory pitch change movements of the blade about its longitudinal axis. The other bearing, which has annular, spherically shaped laminations, also resists centrifugal loads on the rotor blade and accommodates pitch change, flapping and lead-lag movements of the blade. (Lead-lag and flapping movements are oscillatory movements within and perpendicular to, respectively, the plane of rotation of the rotor blades.) Since the pitch change movements of the blade are of a relatively large magnitude (e.g. 10.degree. to 15.degree. or more in each rotational direction), particularly when compared to the lead-lag and flapping motions, the combined torsional movement capabilities of the two laminated bearings must also be relatively large. For economic and space reasons, the bearing having disc-shaped laminations is designed to accommodate a greater proportion of the torsional motion. As a result, the disc-type or thrust bearing is relatively tall or long.
As the height or length of a disc-type laminated bearing is increased, the bearing becomes increasingly unstable in a lateral or radial direction. The lateral instability of a tall disc-type bearing may be accentuated in a rotor blade retention system because the rotor blade, when in motion, tends to move bodily in radial directions relative to its centerline. If only one end of a laminated elastomeric bearing is rigidly connected to the rotor blade, translational shifts of the blade will subject the bearing to shear loads both in and out of the plane of rotation of the blade and rotor, as is explained in the Rybicki 3,759,631 and 3,759,632 patents, for example. Such in-plane and out-of-plane translational shear loads on a laminated bearing are in addition to shear loads exerted on the bearing by pivotal lead-lag and flap motions of the rotor blade.
In the United Aircraft blade retention system, an effort has been made to improve the lateral stability of the constituent disc-type laminated bearing by insuring that opposite ends of the bearing do not move out of axial alignment. The end plates of the disc-type bearing are closely fitted around the root or mounting shaft portion of the associated rotor blade so that translational movements of the blade or pivotal movements of the blade about any axis other than its longitudinal axis do not move the ends of the bearing laterally relative to each other. If the disc-type bearing is to accommodate pitch change movements of the blade, however, one end of the bearing must rotate relative to the other end about the longitudinal blade axis. As shown in the Rybicki 3,759,631, 3,759,632, 3,782,854 and 3,829,239 patents, one end plate of the disc-type bearing, or a rigid member secured to the end plate, slides on and about the circumference of the rotor blade mounting shaft. Such a sliding action necessarily involves a severe wear problem. Although the Rybicki patents show Teflon.RTM. or carbon faced sliding or shear bearings between the end plate of the laminated bearing and the blade shaft, such shear bearings have proved to be only partially successful in eliminating wear for extended periods while maintaining the ends of the laminated disc-type bearing in alignment.
Another version of the United Aircraft retention system, as shown in the Rybicki et al. 3,764,230 patent, incorporates a cylindrical laminated elastomeric bearing between the disc-type thrust bearing and the spherical bearing and in parallel load transmitting relationship with the disc-type bearing. Since the annular or tubular laminations of the cylindrical laminated bearing are concentric with the rotor blade mounting shaft, the radially inner "end" plate or race of the bearing can be fixed to the blade shaft. Pitch change movements of the blade are accommodated through relative rotation between the inextensible layers in the bearing and related shearing of the elastomeric layers. Although the use of such a laminated shear bearing reduces the wear problem associated with a sliding bearing, there remains some possibility of slippage between the inner rigid race of the bearing and the mounting shaft and, more importantly, the prospect of a difficult and expensive effort to replace the bearing when it ultimately requires replacement.
One possible approach to overcoming the difficulties encountered with the United Aircraft rotor blade retention system is to substitute the retention system shown in Gorndt et al U.S. Pat. No. 3,862,812. Like the United Aircraft retention system, the retention system of the Gorndt et al patent utilizes two serially interconnected laminated bearings. In one embodiment of the Gorndt et al system, however, the inboard laminated bearing is a frustroconically shaped laminated bearing, rather than a disc-type laminated bearing. The frustroconically shaped laminated bearing has the lateral stability lacking in the disc-type bearing of the United Aircraft blade retention system. Thus, the only contact necessary between the rotor blade and the two serially interconnected laminated bearings of the Gorndt et al. retention system is at the end of the rotor blade that is connected to the frustroconically shaped laminated bearing. The two laminated bearings are connected through a rigid annular connecting member that encircles the mounting shaft of the rotor blade yet is spaced radially from the shaft.
Although the Gorndt et al blade retention system eliminates the frictional wear problems associated with the United Aircraft blade retention system, the conical bearing of Gorndt et al. must have excessively large outer dimensions in order to accommodate an amount of torsional pitch change movement comparable to the amount of torsional pitch change movement that can be accommodated by the disc-type bearing of the United Aircraft retention system. Moreover, as the outer dimensions of a conical bearing are increased, the torsional spring rate of the bearing is also increased and the bearing becomes more resistant to torsional movement. Thus, as the dimensions of the Gorndt et al. conical bearing are increased to provide a number of elastomeric layers sufficient to accommodate large pitch change movements, the torsional stiffness of the bearing will increase and the spherical laminated bearing will be deflected in preference to the conical bearing during pitch change movements of the rotor blade. The preferential flexing of the spherical bearing will require a large spherical bearing with a resulting increase in torsional stiffness. Attempting to use the Gorndt et al retention system as a substitute for the United Aircraft system will therefore result in a larger, heavier system with a higher overall torsional spring rate.